Sunday, December 2, 2007

2.2.1 The crystalline lens

It is important to understand the biochemistry of the crystalline lens, so we know why cataracts develop and how to prevent them from happening or progressing. Even though cataract surgery is performed as a routine in all developed nations, it is in fact not widely available in less developed countries, simply because people cannot afford to pay for the procedure. The WHO estimates that age-related cataracts constitute 50% of blindness in the World and in 5-10% of people over age 50. And far more women than men are blinded unnecessarily because women live longer and are less likely than men to seek/undergo surgery. Poverty is such an overriding factor!! The key strategy is therefore prevention and inexpensive medical therapy, with surgery reserved as the last resort. And at the same time, no-frills surgery also must be developed, ORs built, and personnel trained.

The crystalline lens is a tiny structure, about 9.5mm in diameter and 4.5mm in thickness. On its front, there is one single layer of cuboidal epithelial cells and the ones in the equatorial region grow into cell fibers that form the cortex. The cortex then merges into adult/juvenile nucleus which overlays the original nucleus developed during the embryonic stage. The ends of cortical cell fibers join to form a Y and an inverted Y "suture" at the front and the back of the lens, respectively. This is why we see all stars (or any point source of light) with 6 points. And the whole epithelium-cortex-nucleus is enclosed in a collagen bag known as the capsule.

The lens actually is present before the immune system develops. That is why mature cataracts must be removed before proteins leak out, to avoid a severe immune response.

So there are two types of cells, the epithelial cells and the cortical/nuclear cell fibers; only the former contain both nuclei and mitochondria. This is a good thing, otherwise when bombarded constantly by ultraviolet rays, there maybe mutations that lead to unpleasant conditions such as tumor of the lens. And in fact, there is no such thing. Further, because the lens is located inside the eye globe, somewhat remote from any source of oxygen, except that dissolved in the aqueous humor, all cells metabolize glucose anaerobically. And the ATP expenditure is mostly for maintaining ionic/osmotic balance by keeping sodium out and potassium in. Any disturbance to ATP production/consumption will have undesirable effects. And one of them is the entry of excess water into the cells which then die.

On the other hand, accommodation, i.e., lens refocusing for near vision, does not seem to require energy. Well, the ATP kind, not the physics kinetic kind. It is a passive lens re-shaping from a change in tension of the zonules, which link lens equator to the ciliary muscle. In other words, near work per se probably does not contribute to energy-related cataract formation. So, relax and read on.